Aspects of this disclosure relate generally to telecommunications, and more particularly to band occupancy techniques for transmissions in unlicensed spectrum.
A wireless communication network may be deployed to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within a coverage area of the network. In some implementations, one or more access points (e.g., corresponding to different cells) provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the coverage of the access point(s). In some implementations, peer devices provide wireless connectively for communicating with one another.
Communication devices in a wireless communications network sending uplink transmissions have assigned resources on the uplink frequency band. In a wireless environment where a wireless local area network (WLAN) device, such as a Long Term Evolution (LTE) device, operates in an unlicensed radio frequency (RF) band shared with a wireless local area network (WLAN) device (e.g., a WiFi device), it may be required that any device actively transmitting on the uplink occupies at least 80% of the bandwidth of a frequency band subframe. However, in many cases, the payload size for an LTE device operating in the unlicensed RF band is very small and cannot meet this requirement. For example, an uplink control channel transmission may regularly occupy only about 1 resource block (RB) per subframe. One solution is to use lower modulation and coding scheme (MCS) and a higher coding gain to generate more coded bits to fill up more modulation symbols. The power spectral density may also be reduced (i.e., reducing transmit power per subcarrier tone), but at the risk of becoming susceptible to interference from other competing devices sharing the bandwidth. Moreover, there may be additional occupancy requirements, such as a spectrum mask requirement, which mandates that within each 1 MHz sliding window, the transmit power cannot exceed certain level.
A wireless terminal device (also referred to as user equipment or access terminal) that needs to transmit on an unlicensed RF band may also be required to perform a clear channel assessment (CCA) each time before sending an uplink transmission. For example, a device may perform CCA/eCCA to determine whether a channel is clear for transmission. Generally, the CCA procedures may involve monitoring a channel for a CCA duration or time slot, for example 20 microsecond (μs). If the time slot is clear (e.g., the communications medium is available or accessible), the device may begin using the channel. When a channel is not clear, the device may initialize a random back-off counter for the channel. Each time the device detects a clear time slot, the random back-off counter is decremented. When the random back-off counter reaches 0, the device may transmit for a limited transmission opportunity. The duration of the transmission opportunity may be a multiple of the CCA time slot duration. During the transmission opportunity, other devices would be blocked by the transmission from also transmitting using the channel. Additionally, other channels on the same device, which are not being used for a transmission, may also be blocked because of RF leakage. In cases where discontinuous uplink transmissions are requested by the terminal device, a failed CCA may cause loss of the channel, and a disruption of the uplink transmission bursts.
An LTE device transmits data in units of resource elements within resource blocks using orthogonal frequency division multiple access (OFDMA) channels for downlink transmissions, and single carrier frequency division multiple access (SC-FDMA) channels for uplink transmissions. The OFDMA and SC-FDMA channels are divided into subchannels within a frequency band and subframes in the time domain. Each subframe is further divided into OFDM symbols to provide data input for FFT and IFFT processing of the carrier frequency. There may be occasions when the LTE device needs a blank OFDM symbol, such as for example to avoid interference by other LTE device transmissions, or to avoid interference to reception of a signal. However, a blank symbol transmission may cause the LTE device to lose access to the channel.
Hence, a mechanism is needed to allow LTE devices to gain access or maintain existing access to unlicensed bandwidth for uplink transmissions while meeting occupancy constraints using unoccupied OFDM symbols, resource blocks, and subframes in the unlicensed bandwidth.